Nonsense! A hard drinker knocks it back! (4)
I believe the answer is:
tosh
'nonsense' is the definition.
(I know that tosh is a type of nonsense)
'a hard drinker knocks it back' is the wordplay.
I cannot really see how this works, but
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'os' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tosh that I've seen before include "13 [TRIPE]" , "Cobblers" , "Pal" , "Hooey" , "Slang nonsense" .)